The sol-gel process is one of the most widely used methods for the preparation of bulk materials and thin films used in integrated optics (IO) circuits.1 The major advantages of the process are its simplicity and its ability to control the purity and homogeneity of the final material on a molecular level. The method offers the possibility of modifying the refractive index, phonon energy, and transparency of a material by choosing suitable matrices like SiO2, TiO2, ZrO2, Al2O3, GeO2, etc.,2-6 either individually or in combination. Such matrices are potential candidates for making planar waveguides, fiber amplifiers, and up-conversion devices, when doped with trivalent lanthanide (also known as rare earth) ions.7-9 Preparation of these matrices involves the direct doping of the materials with Ln3+ ions. The most commonly used lanthanide ion for these applications is Er3+, as it provides amplification in the 1550 nm communication window, through its 4I13/2→4I15/2 transition. Improvements are still needed to optimize performance.
It is desirable to have a high quantum yield and an increased line width for this transition, to enable those materials to be used for broad-band near-infrared amplification. The three main factors that decide the performance characteristics of such lanthanide ion containing materials are the phonon energy of the host in which the lanthanide ions are incorporated, the proximity of the OH groups present in the matrix to the lanthanide ions, and clustering of lanthanide ions. For example in Er3+ incorporated materials, high phonon energy of the host matrix favors the non-radiative relaxation of the 4I13/2 excited state, thereby reducing its life time and quantum yield of the 4I13/2→4I15/2 transition. Because the OH groups, an inherent result of sol-gel process, quench the excited state of the lanthanide ions by dipole-dipole interaction, the proximity of the OH groups to the lanthanide ions, results in a much higher extent of quenching. Finally, clustering of the lanthanide ions reduce the excited state lifetime by concentration quenching.10-11 Several reports are available regarding ways to improve the luminescence characteristics of such materials. These mainly include the works of Biswas et al12-13 and Tanabe et al,14 on the sol-gel glasses and glass ceramics containing Er3+ ions.
Glass-ceramics are usually made by a two step procedure involving the formation of the glass by melting the reagents together at high temperature and quenching, followed by a programmed heat treatment. During the heat treatment, separation of the LaF3 or lanthanide ion doped LaF3 takes place. This method is also not readily applicable to the formation of thin films. Furthermore, these materials have only limited applications as they need to be melted at higher temperature to draw them into fibers. Fiber amplifiers are less convenient for integrated optics because of their increased length and extensive research is going on to replace them with planar waveguide amplifiers.15 A lifetime of 17 ms for the 4I13/2 of Er3+ was reported by Slooff et al.16 for Er3+ ion implanted silica colloidal particles having sizes in the range of 240-360 nm and annealed over the temperature range of 700-900° C. This was attributed to the decreased OH concentration in these materials. The disadvantage of this method is that the ion implantation is a small area, low throughput procedure.
Lanthanide ions like Er3+, Nd3+, etc., have been demonstrated to undergo clustering when incorporated in a silica matrix. Clustered rare earth ions have shorter lifetime compared to the non-clustered ones.
In some matrices some Ln3+ ions are not emissive. For instance, Ho3+ directly doped into SiO2 does not emit light, but via the Ho3+ doped LaF3 nanoparticles they do.
A general method, from readily available starting materials, that combines the advantage of the improved luminescent properties of Ln3+-doped LaF3 nanoparticles and the simplicity of making thin films using sol-gel method, is thus highly desirable.
There is a large interest in cheap efficient generation of (white) light for a variety of purposes such as displays, LCD back light and general lighting appliances. In particular, there is an interest in replacing the incandescent light bulb.17-19 There are three basic approaches to the attainment of white light: i) the conversion of electricity; ii) the conversion of light, either by down-conversion or up-conversion; and iii) thermal radiation in the incandescent lamb to achieve white light.
Electricity is used in light-emitting diodes. There have been some major advances over the last few years in organic light-emitting diodes (OLEDs)20-23 and polymer light-emitting diodes (PLEDs).24-26 However, the generation of white light from OLEDs and PLEDs has proven to be challenging because: 1) blue and white light emitters are not as efficient as green and red emitters;27,28 2) energy down conversion in the case of multilayer devices, i.e. blue light can easily be absorbed by green chromophore and green light can be absorbed by red chromophore which results in one colour emission that depends on their efficiency; 3) bias dependant colour variation i.e. recombination zone of hole and electron is shifted at different bias which leads to different mobility of the charge carriers;29 4) many layers are involved in the multilayer devices which leads to high manufacturing cost;30 and 5) long term stability of emitters such as N—N′-diphenyl-N,N′-bis(3-methyl phenyl)-1,1′-biphenyl-4,4′-diamine (TPD), tris(8-quinolinolato) aluminum (Alq3).31 
Down conversion is the conversion of higher energy UV light into visible light and is widely exploited in phosphors.32 The short-wavelength emitting light sources can be used as efficient pumps to excite organic and inorganic luminescent matrices for subsequent photon emission at lower energies. The main challenge of this process is the degradation of the emitting material, especially in the organic emitting materials, over time because of photodecomposition and other means, as would be known to one skilled in the art.
One of the oldest devices for the production of white light is an incandescent light bulb. An incandescent light bulb produces light by heating a small filament of tungsten to about 2500° C. Despite many years of use, the efficiency (10-12%) is very low.33 
Up-conversion converts cheap near infrared photons via multiphoton processes into visible photons34. Up-conversion is based on sequential absorption and energy transfer steps involving real metastable excited state that is intermediate in energy between the ground state and the emitting state of the ion. This process requires the absorption of at least two photons to provide sufficient energy for the up-converted emission to occur. This process is different from multiphoton absorption process which occurs through the simultaneous absorption of two or more photons via a non-stationary virtual quantum mechanical state in a medium, requiring high excitation densities.
Lanthanide ions are suitable candidates for up-conversion processes because of their crystal field-split (stark) level structure that provides many intermediate levels with favorable spacing and their long-lived excited states. Moreover, cheap NIR diode continuous wave (CW) laser can be used as excitation source.
In order to achieve an efficient, cost effective and durable white light source, the following points may be considered: i) stable photocycle of the emitting species; ii) one cheap excitation source (e.g. 980 nm CW laser) and efficient absorption; iii) easy control over the luminescence intensity of red, green, and blue emission; and iv) easy and cost effective fabrication of the device. It is an object of the invention to overcome the deficiencies in the prior art.